The present invention relates to fiber-optic communications networks, and more particularly, to optical amplifiers with transient control capabilities for use in optical communications networks.
In optical networks that use wavelength division multiplexing, multiple wavelengths of light are used to support multiple communications channels on a single fiber. Optical amplifiers are used in such networks to amplify optical signals that have been subject to attenuation over multi-kilometer fiber-optic links. A typical amplifier may include erbium-doped fiber amplifier components that are pumped with diode lasers. The erbium-doped fiber amplifier stages increase the strength of the optical signals being transmitted over the fiber-optic links.
The gain of the erbium-doped fiber amplifier stages depends on the inversion level of erbium ions in the fiber. If, for example, the inversion level of a given stage is high, the gain of the stage will be high. If the inversion level of a stage is low, the gain of the stage will be low. Unless control electronics are used to maintain a steady inversion level under various operating conditions, the gain of erbium-doped fiber amplifier stages will be subject to unacceptable transients. Gain transients in an amplifier may cause fluctuations in the power of the output signals from the amplifier. If the output signals are too weak, it may not be possible to detect the signals. If the output signals are too strong, the signals may be subject to nonlinear optical effects in the fiber.
It is an object of the present invention to provide optical amplifier systems in which gain transients are controlled.
This and other objects of the invention are accomplished in accordance with the present invention by providing optical amplifiers that use taps to monitor optical signal powers. The amplifiers may contain one or more diode-laser-pumped rare-earth-doped fiber coils for providing optical gain. Optical filters may be used to modify the optical signal powers that have been tapped.
The input power to an amplifier may be filtered using a spectrum filter that matches the gain spectrum of the rare-earth-doped fiber coils. This allows a feed-forward approach to be used to control the pump power for the coils, even if the gain spectrum of the coils is not flat.
The output power from the amplifier may be filtered using a spectrum filter that flattens the overall gain spectrum of the amplifier. This allows a feedback approach to be used to control the pump power for the coils, even if the gain spectrum of the amplifier is not flat.
A combination of feed-forward and feedback techniques may be used to control amplifier transients. Spectrum filters may be used on both input and output taps.
Further features of the invention and its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.